We used our time on beamline 7-1 to collect data from E. coli ribonuclease I (RNase I), a base-nonspecific endoribonuclease. RNase I is a member of a widely distributed group of ribonucleases found in bacteria, fungi and plants. The gene for RNase I has been cloned and the enzyme has been subjected to site-directed mutagenesis. During this beamtime period we collected data to 2.0 E resolution from native crystals and Pt and U heavy-atom derivatives. In all cases the crystals were complexed with a DNA oligomer which is believed on chemical evidence to bind to the enzyme's active site. (We use DNA oligomers because RNA would be cut up by the enzyme). The data have been processed and have been used to calculate an electron density map. Refinement is in progress; as of now (Nov. '98) we have a 2.0 E resolution map with a crystallographic R factor of 22%. Refinement is continuing.